1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a thin magnetic metal film as a magnetic recording layer, and particularly to a magnetic recording medium provided with an improved protective layer.
2. Description of the Prior Art
As magnetic recording media such as audio tapes and video tapes, there have been widely used coated-type media which are obtained by coating a non-magnetic support with a powdery magnetic material such as a magnetic oxide or a ferromagnetic alloy in an organic binder, followed by drying. Recently, however, much attention has been paid to the use of thin metal film type magnetic recording media, in place of the coated-type media, as practical media for high-density recording. The thin metal film type magnetic recording medium has a thin film of a ferromagnetic metal, e.g., a Co-Ni alloy provided on a support as a magnetic recording layer. A magnetic recording layer of this type is formed from a ferromagnetic metal having a saturation magnetization higher than that of a magnetic oxide powder or the like, without the use of any binder or the like; therefore, such a layer is very advantageous from the viewpoint of electromagnetic conversion characteristics and is expected to realize high-density recording.
However, the thin metal film type magnetic recording media encounter the following problems: (1) since a magnetic recording medium is placed in frictional contact with the magnetic head, guide poles, etc. during recording, reproduction and erasing of magnetic signals, and in particular a video tape is subjected to repeated mechanical shocks in reproducing a still image, the magnetic recording medium should have a low coefficient of friction, high abrasion resistance and high scratch resistance, but the conventional thin metal film type magnetic recording media have low coefficients of friction with the magnetic head, etc. and are low in both abrasion resistance and scratch resistance, so that the magnetic recording layer thereof is liable to be damaged; (2) the thin metal type magnetic recording media are easily corroded by adverse environmental conditions due to their low weather resistance.
To overcome the abovementioned problems, attempts have been made to provide a protective film on a thin metal film type magnetic recording medium. For this purpose, a method of applying a film of organic lubricant on the thin metal film (Japanese Laid-open Patent Publication No. 75001/1975) and a method of providing a protective film of a lubricant such as a metal or a metal oxide on the thin metal film (Japanese Laid-open Patent Publication No. 39708/1978 and No. 40505/1978) have been proposed. However, although these methods are effective to some extent, the effects are not long-lasting and, accordingly, the methods are not practical.
Another method has been proposed in which the magnetic metal layer is coated with a polymeric substance such as a copolymer of vinylidene chloride and an acrylic acid ester (Japanese Laid-open Patent Publication No. 155010/1979). However, since the coating has a large thickness of about 0.2 .mu.m, the spacing loss will be great, which causes a reduction in output and makes the magnetic recording medium unsuitable for high-density recording.
Still another method has been proposed in which a plasma-polymerized film of a single-layer construction is provided as a protective layer (Japanese Laid-open Patent Publication No. 135442/1982 and No. 102330/1983). This method makes it possible to form a protective film which is relatively tough and has good adhesion to the thin magnetic metal film. In addition, a protective film having an extremely small thickness of 0.1 .mu.m or less can be provided, so that the spacing loss can be reduced to a negligible level. However, conventional protective films formed by plasma polymerization have been unable to enhance both scratch resistance and weather resistance of the thin metal film type magnetic recording media.